A long lost light reflector left on the lunar surface by the Soviet Union nearly 40 years ago has been found by a team of NASA physicists.

The French-built laser reflector was sent aboard the unmanned Luna 17 mission, which landed on the moon November 17, 1970, releasing a robotic rover that roamed the lunar surface and carried the missing laser reflector. The Soviet lander and its rover, called Lunokhod 1, were last heard from on September 14, 1971.

The discovery of the Soviet reflector came as a surprise, because scientists had actively searched for it for nearly four decades without success. Many scientists had speculated that the Lunokhod 1 rover might have fallen into a crater or parked badly, with its reflector not facing the earth, which would have prevented it from being located by laser pulses.

Soviet robotic lander Luna 17 still sitting on Mare Imbrium where it delivered the Lunokhod 1 Rover in November 1970. (Photo Credit: NASA/GSFC/Arizona State University)

“No one had seen the reflector since 1971,” said Tom Murphy, an associate professor of physics at UCSD. Murphy heads a long-term effort looking for deviations of Einstein’s theory of general relativity by measuring the shape of the lunar orbit to within an accuracy of one millimeter. They measure the shape of the orbit by timing the reflections of pulses of laser light from reflectors left  by the Apollo astronauts and turning the timing measurement into a distance.

Three reflectors are required to lock down the orientation of the moon. A fourth adds information about tidal distortion of the moon, and a fifth enhances that information.

“Lunokhod 1, by virtue of its location, would provide the best leverage for understanding the liquid lunar core, and for producing an accurate estimate of the position of the center of the moon—which is of paramount importance in mapping out the orbit and putting Einstein’s gravity to a test,” said Murphy.

Lunokhod rovers were about 2.3 meters long and 1.5 meters tall.
(Photo Credit: NASA)

Researchers found the rover last month when the high-resolution camera on NASA’s Lunar Reconnaissance Orbiter, or LRO, obtained images of the landing site. The camera team identified the rover as a sunlit speck on the image—miles from where the team had been searching. Until now the existence of the reflector or its precise location was unknown.

“It turns out we were searching around a position miles from the rover,” said Murphy. “We could only search one football-field-sized region at a time. The recent images from LRO, together with laser altimetry of the surface, provided coordinates within 100 meters.

On April 22, researchers sent pulses of laser light from the 3.5 meter telescope at the Apache Point Observatory in New Mexico, zeroing in on the target coordinates provided by the LRO images. They found the Lunokhod 1 reflector and pinpointed its distance from earth to within one centimeter. A second observation less than 30 minutes later allowed the team to triangulate the reflector’s exact spot on the moon, to within 10 meters. In the coming months, Murphy estimates it will be possible to establish the reflector’s coordinates to better than one-centimeter precision.

The return signal from the reflector was measured as a collection of individual particles, or photons, of laser light.

“We quickly verified the signal to be real and found it to be surprisingly bright: at least five times brighter than the other Soviet reflector, on the Lunokhod 2 rover, to which we routinely send laser pulses,” Murphy said. “The best signal we’ve seen from Lunokhod 2 in several years of effort is 750 return photons, but we got about 2,000 photons from Lunokhod 1 on our first try. It’s got a lot to say after almost 40 years of silence.”

Murphy and his colleagues found in a study they published this month that lunar dust may be obscuring the reflectors on the moon.

“Near full moon, the strength of the returning light decreases by a factor of ten,” he adds. “We need to understand what is causing this if we are contemplating putting additional scientific equipment on the moon. Finding the Lunokhod 1 reflector will add important clues to this study.”